Pure syndiotactic 1,2-polybutadiene (SPB) is a thermoplastic resin which has double bonds attached in an alternating fashion to its polymeric backbone. Films, fibers and molded articles can be made utilizing syndiotactic 1,2-polybutadiene. It can also be blended into rubbers and cocured therewith. Syndiotactic 1,2-polybutadiene can be made by solution, emulsion or suspension polymerization. It typically has a melting point which is within the range of about 195.degree. C. to about 215.degree. C. Many transition metal catalyst systems based on titanium, vanadium, chromiumm, molybdenum, palladium, iron and cobalt have been reported in the prior art for the preparation of syndiotactic 1,2-polybutadiene (see, i.e., J. Boor, Jr., Ziegler-Natta Catalysts and Polymerizations, Academic Press: New York, 1979, P. 144). However, the majority of these catalyst systems have no industrial application because they have insufficient polymerization activity and stereoselectivity and in some case produce low molecular weight polymers or cross-linked polymers unsuitable for commercial use. The following cobalt-containing catalyst systems have been widely known for the preparation of syndiotactic 1,2-polybutadiene:
I. Cobalt dibromide/triusobutyl aluminum/water/triphenyl phosphine disclosed in U.S. Pat. No. 4,182,813, assigned to Japan Synthetic Rubber Co. Ltd.; and, PA1 II. Cobalt tris(acetylacetonate)/triethyl aluminum/water/carbon disulfide disclosed in U.S. Pat. No. 3,778,424, assigned to Ube Industries Ltd.
These two catalyst systems also have serious disadvantages. The cobalt dibromide/triisobutyl aluminum/water/triphenyl phosphine system yields syndiotactic 1,2-polybutadiene having low crystallinity. In addition, this catalyst system develops sufficient catalytic activity only in halogenated hydrocarbon solvents as polymerization medium, and halogenated solvents present the problems of toxicity. The second catalyst system uses carbon disulfide as one of the catalyst components, thereby necessitating the use of special safety measures due to its high volatility, low flash point as well as toxicity. Accordingly, many restrictions are required for the industrial utilization of the said catalyst system. Furthermore, the syndiotactic 1,2-polybutadiene produced with this catalyst system has a very high melting point (200.degree. C. to 210.degree. C.) and is therefore difficult to process.
Coordination catalysts based on chromium compounds such as triethylaluminium/chromium tris(acetylacetonate) have a low activity and have not been usable on a technical scale. JP-A-7306939 and JP-A-7364178, both assigned to Mitsubishi, disclose a process for polymerization of 1,3-butadiene to amorphous 1,2-polybutadiene by using a ternary catalyst system comprising (A) a soluble chromium compound, (B) a trialky aluminum compound, and (C) a dialkyl hydrogen phosphite. The product was reported to be a white rubbery polymer containing a portion of gel and displaying no obvious melting point.
U.S. Pat. No. 4,751,275, assigned to Bayer, discloses a process for the preparation of SPB by the solution polymerization of 1,3-butadiene in a hydrocarbon polymerization medium, such as benzene, toluene, cyclohexane, or n-hexane. The catalyst system used in this solution polymerization contains a chromium-III compound which is soluble in hydrocarbons, a trialkylaluminum compound, and di-neopentylphosphite or neopentylmethylphosphite. However, the polymerization product was not well charaterized as neither the melting temperature nor the degree of syndiotacticity is reported.
U.S. Pat. No. 4,168,357 and U.S. Pat. No. 4,168,374, assigned to Goodyear, describe chromium-containing catalysts for the preparation of high cis-1,4-polypentadiene.
Notwithstanding the foregoing prior art, it would be advantageous to produce a new and improved catalyst system that can be used to produce a syndiotactic 1,2-polybutadiene product having a higher melting point and increased syndiotacticity compared to the syndiotactic 1,2-polybutadiene produced by the processes of the prior art.
The object of this invention is to overcome the disadvantages of the prior art and provide a new and improved catalyst system for the production of syndiotactic 1,2-polybutadiene having a higher melting point and increased syndiotacticity over syndiotactic 1,2-polybutadiene products of the prior art.